The basic theory and operation of thermoelectric devices have been developed for many years. Thermoelectric devices may function as coolers and/or heaters. Thermoelectric devices are essentially small heat pumps that follow the laws of thermodynamics in the same way as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. Thermoelectric devices, however, function with solid state electrical components as opposed to more traditional mechanical, fluid, heating and cooling components.
An assembly for a simple thermoelectric device generally includes two dissimilar materials such as N-type and P-type thermoelectric semiconductor elements. Heating and cooling with a thermoelectric device occurs by arranging the thermoelectric elements in an alternating N-element and P-element electrical configuration, with the thermoelectric elements electrically coupled in series and thermally in parallel. The Peltier effect occurs in the thermoelectric devices when a DC voltage applied to the N-type and P-type elements results in current flow through the serial electrical connection and heat transfer across the N-type and P-type elements in the parallel thermal connection. In a typical thermoelectric element array, the direction of net current flow through the thermoelectric elements determines the direction of heat transfer.
Previously developed thermoelectric systems are generally designed for use with a given power source, e.g., 12 volts or 24 volts. The number of thermoelectric elements within a thermoelectric device determines the operating voltage for the thermoelectric device. Therefore, previously developed thermoelectric systems are generally either 12 volt or 24 volt systems, and so long as the appropriate operating voltage is applied to the thermoelectric system, the desired heating or cooling is achieved.
Portable thermoelectric systems, such as portable coolers or heaters, have been previously developed for use in vehicles. These thermoelectric systems can generally be coupled directly to a vehicle's electrical system since most vehicle electrical systems typically provide a DC voltage. Currently, many vehicles have a 12 volt DC electrical system allowing for coupling a 12 volt thermoelectric system directly to the vehicle's electrical system.
A growing number of vehicles, particularly trucks in Europe and Asia, have 24 volt systems. Since many existing thermoelectric systems are compatible with only 12 volts, the 24 volts provided by these vehicle's electrical systems must be reduced to run a 12 volt thermoelectric system. Prior approaches to making a 12 volt thermoelectric system compatible with a 24 volt power source generally involve converting the 24 volts to 12 volts by DC to DC conversion. DC to DC conversion is often less than completely efficient and may result in voltage losses. For example, converting 24 volts to 12 volts may actually only provide 10 or 10.5 volts for a 12 volt thermoelectric system. Applying less than 12 volts to a 12 volt thermoelectric system may prevent the thermoelectric system from achieving designed cooling or heating capacity and may also damage the components within the thermoelectric system.
Another previously developed approach for providing a thermoelectric system compatible with multiple power sources has been to include multiple thermoelectric assemblies within the thermoelectric system with each assembly rated for a different operating voltage. In this way, for example, a thermoelectric system may include a 12 volt thermoelectric assembly and a 24 volt thermoelectric assembly. The appropriate thermoelectric assembly is activated based on the available operating voltage. Multiple thermoelectric assemblies in a thermoelectric system have disadvantages of adding additional expense to the thermoelectric system as well as requiring additional space.
Previously developed thermoelectric systems sometimes also implement inappropriate temperature regulating techniques. Previously developed systems may provide temperature regulation by turning a thermoelectric assembly on and off as predetermined temperatures are reached. This on and off approach to temperature regulation may result in undesired temperature cycling of the thermoelectric device. The different coefficients of thermal expansion for the materials in a thermoelectric device may cause thermally-induced mechanical stresses in the thermoelectric device when the device is temperature cycled. These stresses can damage the device. Therefore, excessive temperature cycling of a thermoelectric device may reduce its reliability and service life.
Previously developed thermoelectric systems also do not have a power saving operating mode. Previously developed thermoelectric systems are generally either on or off and do not provide for operating at less than full power. This may present a needless drain on the power source that diminishes its service life, particularly when the power source is a battery.